The present invention relates to photoactive monomers, to oligomeric intermediates containing said monomers, to photoreactive polymers containing said monomers useful as positive acting photoresists, and to methods for making said monomers, intermediates, and polymers.
Photoreactive polymers are useful as binder resins in photoresist compositions employed in photodevelopment of electronic components such as circuit boards and other products. Circuit boards are manufactured in a number of processing steps which rely on the use of photoreactive coatings (or photoresists) that photochemically produce a difference in solubility between the photoexposed areas and the unexposed areas. In general, two classes of photoresist exist: positive acting resists and negative acting resists. A positive acting resist becomes more soluble in a developer solution when exposed to actinic radiation, and a negative acting resist becomes less soluble in a developer solution when exposed to actinic radiation. For many applications a positive acting resist is preferred. An object of the present invention is to provide novel positive acting photoresists.
One situation in which positive acting resists are preferred is in the case of circuit boards that have through holes that permit connection of one board to an adjacent board in a stack. These through holes are copper coated and must be protected from etchants. One method to accomplish this is the use of an applied, preformed film which covers the hole and protects the copper from etchants during processing. A more recent development is the electrodeposition of photoresist, and this approach has significant advantages over applied film for coating the copper in the holes with photoresist without plugging. An objective has been to create a positive acting, electrodepositable photoresist which could coat the hole, protecting it from etchants, and then be removed from the hole more easily than negative photoresists. Negative acting resists have disadvantages for protecting through holes because of the inherent difficulties associated with removing a crosslinked material from a small space such as a through hole. Furthermore, there is difficulty in exposing negative photoresist material that is located within a hole in order to crosslink such a resist so that it can protect the copper. With a positive photoresist, on the other hand, the holes need not be exposed since the resist material in the holes does not need to become crosslinked in order to serve its purpose.
Diazo functional moieties such as quinonediazidesulfone derivatives having structures (1) and (2) in which R is typically chlorine (e.g., sulfonyl chloride) ##STR1## are well known as photoreactive groups for use in positive acting photoresists. In the synthesis of those prior art compounds, sulfonyl chloride is condensed with hydroxyl or amino functionalities attached to monomeric, oligomeric, or polymeric materials. The quinonediazidesulfone derivatives in such a photoresist function by photochemically generating an intermediate ketene which reacts with water to form a carboxylic acid. Photoexposed areas contain salt-forming carboxylic acid groups which dissolve in basic developing solutions. Dissolution of unexposed area in a basic developer is inhibited by the presence of the unreacted hydrophobic components (1) or (2). If water is not present the ketene will react with other hydroxyl groups to form undesirable esters which are not subject to solubilization by a developer. Since the photoreaction mechanism requires the presence of water to work well, a burden is imposed on the user to process the circuit boards under carefully controlled conditions so that the boards all undergo exactly the same dehydration bakes and are handled in very carefully controlled humidity conditions. It would be desirable to have available alternative chemistry for positive acting photoresists that would not entail such precautions.
Many of the prior art photosensitive groups for positive photoresists include molecular groups that are hydrolytically sensitive, which limits the versatility of these groups for use in electrodepositable formulations, whether cationic or anionic. As reported in U.S. Pat. Nos. 5,166,036; 4,975,351 and 5,134,054 the storage stability of electrodepositable photoresists based on diazo containing materials is poor and is attributed to hydrolyric instability of the sulfonyl linkage. Examples of other hydrolytically unstable groups include acetals, polyesters, t-butoxycarbonyl (t-BOC) protected carboxylates or phenols, and sulfonate esters. When a cationic or anionic dispersion is electrodeposited on a conductive substrate, a pH of 12 to 14 or 1 to 2, respectively, may be created at the interface of the coating and the substrate. A pH of 12 to 14 may be created in the case of a cationic coating. It is well known that diazo functionalities are sensitive to both high and low pH conditions and will react to form undesirable reaction products. The other chemistries such as t-BOC protected groups, acetals, and esters are also subject to hydrolysis under certain conditions of high and low pH, especially under aqueous conditions. Furthermore, stability of the chemistry under coating conditions and post-coating bake conditions is often given little or no consideration in the prior art. After a substrate has been electrocoated it is usually necessary to bake the coating for a sufficient time to allow for complete coalescence as well as evaporation of water and any volatile organic components. In the case of heat-sensitive diazo functional materials, even short bake times at high temperatures can decompose the diazo compounds. The use of long bake times at lower temperatures severely reduces the processing speed for a manufacturer.
The irradiation of photoresist, in the case of circuit board manufacture, often occurs through a glass or plastic cover sheet. Radiation passing through such a cover sheet to reach the photoresist is predominantly that having wavelengths greater than approximately 315 nanometers. The principal wavelength used for irradiation of photoresists is the 365 nanometer wavelength of a mercury vapor ultraviolet lamp. Therefore, a useful photoresist for printed circuit board manufacture is preferably sensitive to radiation having wavelengths greater than 315 nanometers, particularly to radiation in the vicinity of 365 nanometers.
Some prior art approaches to electrodepositable, positive photoresist rely on photo-generated solubilizing groups which are pendant to the main polymer chain of the photoresist polymer. The theoretical maximum quantum efficiency (the number of reactions divided by the number of photons impinging on the photoresist) of such a system is one, i.e., each photon entering the photoresist would ideally result in formation of a solubilizing group. However, the quantum efficiency is usually much less than one. In order to overcome this limitation on quantum efficiency, systems have been developed which rely on photogenerated catalysts so that one photoreaction produces one catalyst which promotes many other reactions. U.S. Pat. No. 5,230,984 uses photogenerated acid catalysts generated by exposures of 800 millejoules per square centimeter, which is a relatively high exposure dosage. Higher photosensitivity permitting lower exposure dosages would be desirable. Also, these prior art systems require a bake following photoexposure, which undesirably increases processing time. The use of a catalyst can also hurt resolution by diffusion into the surrounding polymer and causing reactions outside of the desired regions. Known photo generated catalysts are based on sulfonate esters of2,6-dinitro-p-xylene.
A wide variety of nitrobenzyl alcohol structures are theoretically encompassed by generic structures in Japanese Patent Applications 63-146029, 03-131626, 03-141357, and 63-247749. These applications disclose nitro-containing benzyl alcohol derivatives specifically for use in applications employing short wavelength ultraviolet radiation in the region of 248 nanometers. They fail to recognize the surprisingly high photosensitivity at longer wavelengths (particularly 365 nanometers) of certain dinitrobenzyl structures. Furthermore the above-numerated Japanese applications are non-enabling as to a synthesis for the particular dinitro structures of the present invention. The syntheses disclosed in the Japanese publications for other species would not be suitable for producing the dinitrobenzyl alcohols of the present invention at practical yield levels. Furthermore, these applications fail to instruct the use of polymers derived from this material in electrodepositable compositions. In fact, the types of polyesters disclosed would be expected to be hydrolytically unstable due to the presence of ester groups.